This invention relates to three-dimensional imaging, and, more particularly, to a method and apparatus for defining a three-dimensional imaging section.
Three-dimensional imaging is commonly employed to allow an operator to obtain three-dimensional images that show the interior of a structure of interest. A common application of three-dimensional imaging is medical imaging, and a common technique for performing three-dimensional imaging, especially in the context of medical imaging, is magnetic resonance imaging.
Magnetic resonance imaging is one example of a variety of techniques used for medical imaging. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field Bz), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment M1, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment Mz is tipped, and hence the magnitude of the net transverse magnetic moment M1 depends primarily on the length of time and the magnitude of the applied excitation field B1. A signal is emitted by the excited spins, and after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing MRI to produce images, a technique is employed to obtain MRI signals from specific locations in the subject. Typically, the region which is to be imaged is scanned by a sequence of MRI measurement cycles which vary according to the particular localization method being used. The resulting set of received MRI signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, MRI signals are elicited from specific locations in the subject. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) which have the same direction as the polarizing field B0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each MRI cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting MRI signals can be identified.
MRI data for constructing images can be collected using one of many available techniques, such as multiple angle projection reconstruction and Fourier transform (FT). Typically, such techniques comprise a pulse sequence made up of a plurality of sequentially implemented views. Each view may include one or more MRI experiments, each of which comprises at least an RF excitation pulse and a magnetic field gradient pulse to encode spatial information into the resulting MRI signal.
In order for an MRI system to acquire signals from a region of interest to an operator, the operator first defines or prescribes the acquisition that is to be performed, including inputting parameters pertaining to the field of view as well as the orientation of the desired image or images. In general, it is desirable for the operator to be able to optimally perform the prescription such that the prescribed image accurately encompasses the region of interest. It is undesirable for the prescribed image not to include all the structure that is of interest to the operator. However, if the prescription is performed such that the prescribed region is too large, then the resolution with which information is acquired for the region of interest is unnecessarily reduced.
In order to provide an improved method of defining a three-dimensional imaging section, one embodiment of the invention provides a method which comprises displaying a plurality of localizer images of the structure of interest, acquiring operator inputs that designate regions on the plurality of localizer images and that correspond to regions within the structure of interest, and determining a volume based on the regions designated by the operator inputs. The volume defines the three-dimensional imaging section, which is a three-dimensional section of a structure of interest.
Another embodiment of the invention provides an imaging system comprises a graphic prescription interface and a processor. The graphic prescription interface includes a plurality of localizer images and a plurality of prescription marks. The plurality of prescription marks are displayed on the plurality of localizer images in response to operator inputs. The prescription marks include first, second, third and fourth prescription marks that correspond to respective points within a structure of interest. Each of the first, second, third and fourth prescription marks are displayed on a particular one of the plurality of localizer images that is selected to be indicative of a first coordinate of a respective point, and each of the first, second, third and fourth prescription marks is displayed on the selected localizer image at a location that is indicative of second and third coordinates of the respective point. The processor is coupled to receive information pertaining to the first, second and third coordinates for each of the respective points corresponding to the first, second, third and fourth prescription marks, and is adapted to use the information to determine a volume that defines a three-dimensional imaging section which encompasses the respective points corresponding to the first, second, third and fourth prescription marks.